Myocyte cell loss is a prominent and important pathogenic feature of cardiac ischemia (myocardium infarction). It is widely known that a great amount of cardiac cell loss after infarction happens by a mechanism called ischemia/reperfusion (IR) injury. The IR injury allows for massive cell death to happen only after the blood supply (thus oxygen supply) is reestablished after cardiac ischemia (thus hypoxia) triggers cell death. This indicates that normoxic condition after hypoxia is a key for massive cardiac cell loss after myocardium infarction. Limiting this loss is a desirable therapeutic goal, but the development of truly effective strategies to achieve that goal requires an understanding of the mechanisms by which ischemia triggers cell death. Investigators have turned to isolated and cultured cardiomyocytes to identify signaling pathways involved in the response to ischemia and to systematically test the effectiveness of pro survival signaling pathways and various anti-death molecules against ischemia-associated cellular insults, such as hypoxia.
Reperfusion is the most effective strategy to save the ischemic tissue, but it can cause additional damage, leading to cell dysfunction and death. The pathology of ischemia-reperfusion (IR) injury has been observed in the heart, brain, liver, and kidney.
BNIP3 stands for Bcl-2 and nineteen-kilodalton interacting protein-3, and is a member of the Bcl-2 protein family. Bcl-2 proteins have been implicated in the control of both apoptotic and necrotic cell death and in guarding mitochondrial integrity. They share up to 4 conserved regions of homology known as Bcl-2 homology domains (BH1, BH2, BH3, and BH4), which mediate interactions among the various family members, and are divided functionally into antiapoptotic and proapoptotic members. Many of these proteins normally reside in membranous cellular structures, including mitochondria, endoplasmic reticulum, and the nuclear envelope or are recruited to such structures (principally the mitochondria) during the execution of cell death signaling pathways. Antiapoptotic members, such as Bcl-2, Bcl-XL, Mcl-I, AI, Bcl-W, display sequence homology throughout all 4 BH domains. Proapoptotic members that antagonize the activity of many pro survival proteins and induce cell death when overexpressed, display homology to fewer BH domains. Some, like Bax and Bak, contain BH1, BH2, and BH3 domains, whereas many others (Bad, Bid, Bik, Bim, BimL, Blk, and Noxa) possess only the BH3 domain (BH3-only proteins).
BNIP3 is the founding member of small group of BH3-only proteins that includes BNIP3, NixIBNIP3L, and BNIP3H. In contrast to Bid and Bad, the proapoptotic activity of BNIP3 and Nix is regulated through transcriptional mechanisms that involve the HIF complex. Thus, the promoter for BNIP3 contains a functional binding site for the HIF transcriptional complex (hypoxia response element, HRE) and its mRNA and protein expression are dramatically increased in multiple cell types in response to reduced oxygen concentration. In cultured cells, increased expression of BNIP3 appears to be part of a second wave of hypoxia induced protein accumulation, occurring late relative to other well-characterized HIF-inducible genes that are involved in promoting angiogenesis, glycolytic metabolism, and survival (e.g. erythropoietin, VEGF, heme oxygenase, hexokinase, and IGF2).
It has been shown that (i) BNIP3 expression is dramatically increased in response to hypoxia, (ii) enforced expression of BNIP3 causes cell death in normoxic cardiomyocytes, and (iii) enforced expression of a BNIP3 mutant lacking its transmembrane domain (BNIP3L1TM) partially blocks hypoxia-induced cell death.
Therefore, there still exists a need for novel methods and drugs to mitigate or prevent IR induced cell death in cardiac myocytes as a result of cardiac arrest, infarct, or due to surgical intervention.